External beam radiation treatment is a non-invasive treatment method for pathological anatomies such as benign or malignant tumors, lesions and arteriovenous malformations (AVMs), which use a precisely positioned radiation beam to necrotize pathological tissue.
In one type of external beam radiation treatment, an external radiation source is mounted in a gantry that is rotated around a center of treatment (isocenter) and directs a sequence of x-ray beams at a pathological anatomy from multiple angles, with the patient positioned so the pathological anatomy is at the isocenter. As the angle of the radiation source changes, every beam passes through the pathological anatomy, but passes through a different area of healthy tissue on its way to the pathological anatomy. As a result, the cumulative radiation dose at the pathological anatomy is high and the average radiation dose to healthy tissue is low. In some systems, the radiation source includes a multi-leaf collimator (MLC) that may be used to shape the radiation beam.
In another type of external beam radiation treatment (e.g., the CYBERKNIFE® Robotic Radiosurgery System manufactured by Accuray Incorporated of Sunnyvale, Calif.), the radiation source is mounted on a robotic control arm with multiple degrees of freedom, allowing the treatment to be non-isocentric to achieve better dose conformality and homogeneity relative to isocentric systems.
The application of either type of treatment (i.e., isocentric or non-isocentric) is preceded by a diagnostic and treatment planning phase where a medical physicist determines the appropriate radiation dose for the pathological anatomy and plans the sequence of radiation treatment beams (e.g., position, location, angle, duration and shape) to achieve the prescribed dose.
In forward treatment planning, the medical physicist determines parameters such as the trajectory and duration of the radiation beams to be applied to a pathological anatomy and then calculates how much radiation will be absorbed by pathological tissue, critical structures (i.e., vital organs) and other healthy tissue. The parameters describing the beams may then be successively updated by the physicist until the radiation dose distribution is deemed acceptable.
In inverse planning, in contrast to forward planning, the medical physicist specifies the minimum dose to the tumor and the maximum dose to other healthy tissues independently, and the treatment planning software then selects the direction, distance, and total number and energy of the beams in order to achieve the specified dose conditions.
Conventional treatment planning systems are designed to import three-dimensional (3D) images from a diagnostic imaging source such as computerized x-ray tomography (CT) scans. CT is able to provide an accurate three-dimensional model of a volume of interest (e.g., skull or other region of interest of the body) generated from a collection of CT slices and, thereby, the volume requiring treatment can be visualized in three dimensions.
For most applications in radiosurgical treatment planning, it is sufficient to delineate anatomical structures on planar two-dimensional (2D) slices of 3D CT image volumes, with the possible additional steps of viewing renderings of the structures in the space of the 3D volumes during or after the delineation step. However, for some applications, such as treating cranial arteriovenous malformations (AVMs), for example, 3D CT images are not always sufficient for target delineation.
An AVM is a congenital disorder of the connections between veins and arteries in the vascular system. Normally, the arteries in the vascular system carry oxygen-rich blood at a relatively high pressure. Structurally, arteries divide and sub-divide repeatedly, eventually forming a sponge-like capillary bed. Blood moves through the capillaries, giving up oxygen and taking up waste products from the surrounding cells. Capillaries successively join together, one upon the other, to form the veins that carry blood away at a relatively low pressure.
In an AVM, the arteries are connected directly to the veins in a tangled interconnection and the capillary bed is missing. The tangle of blood vessels forms a relatively direct connection between high pressure arteries and low pressure veins. This collection of blood vessels, known as a nidus, can be extremely fragile and prone to bleeding. AVMs can occur in various parts of the body including the brain, where bleeding can cause severe and often fatal strokes. If detected before a stroke occurs, the AVM can be treated with external beam radiation. The radiation damages the walls of the veins and arteries of the nidus. In response, the walls thicken and grow in, eventually closing off the arteries feeding blood into the nidus.
With respect to AVMs, one of the goals of treatment planning is to identify the nidus of the AVM and to distinguish it from its feeding vessels. However, identifying the nidus and its feeder vessels in a CT scan is difficult because the target vasculature has very low contrast in the x-ray modality of CT scans. In order to visualize the AVM, including the nidus and the feeding vessels, the patient can be injected with an x-ray contrast agent immediately prior to CT imaging. However, because of the technical limitations on image acquisition speed of 3D CT images, the 3D images generally show the AVM after the contrast agent has suffused the nidus. While it is sometimes possible to delineate the nidus from the 3D images, it may often be difficult to distinguish the feeding vessels from the nidus and to identify the boundary between the nidus and the feeding vessels.
As an alternative, the patient may be imaged in a separate 2D angiographic imaging system, which may include a fixed x-ray source and detector or, alternatively, a source and detector that are movable around the patient to capture different views. Images can be acquired both before and after the injection of the contrast agent. The ‘before’ image can be subtracted from the ‘after’ image to produce a difference image known as a digital subtraction angiography (DSA) image.
In order to distinguish the feeding vessels from the nidus, a rapid series of fixed, 2D x-ray projection images can be taken from the time the contrast agent is injected until it enters the nidus. The 2D images can then be examined after the fact to show the contrast agent advancing through the feeding vessels and entering the nidus. The image that best distinguishes the feeding vessels from the nidus can then be selected from the sequence.
In order for the 2D angiograms to be useful for radiosurgical treatment planning, they need to be integrated with the 3D CT scan data. However, the imaging geometry of the angiographic imaging system (e.g., imaging angles and source and detector separations) may be unknown with respect to the imaging geometry of the CT imaging system, so that the two sets of images cannot be directly integrated. Conventionally, in the case of cranial AVMs, the patient is fitted with an invasive frame that holds a configuration of fiducial markers. The attachment points of the frame are sharply pointed screws that pierce the skin and enter the skull of the patient. The fiducial markers then appear as landmarks in the angiographic images. The frame remains attached to the patient during a subsequent CT scan so that the landmarks appear in the CT images. Different slices of the CT image can then be iteratively compared with the angiographic images to find a matching orientation. The frame may also be required for patient alignment during treatment, requiring the patient to suffer the discomfort of the invasive frame continuously through the process of diagnostic imaging, treatment planning and treatment delivery.